The Far Infrared Absorption Spectra of Ammonia and Phosphine Gases under High Resolving Power

A spectrometer of large aperture employing a reflection grating of the echelette type ruled to concentrate radiation in the region of the 1st order of 90μ has been applied in the spectral range from 60 to 125μ. The resolving power was several times that hitherto obtained in this region. The pure rotation absorption spectra of water vapor and of the gases N${\mathrm{H}}_3$ and P${\mathrm{H}}_3$ have been investigated. In the case of N${\mathrm{H}}_3$ the absorption lines shown as single in the observations of Badger and Cartwright have been found under the higher resolution of the present work to be doublets with a doublet separation of 1.33 ${\mathrm{cm}}^{-1\mathrm{}}$. Despite the similarity of structure of the molecules P${\mathrm{H}}_3$ and N${\mathrm{H}}_3$ the pure rotation lines of P${\mathrm{H}}_3$ exhibit no trace of a doubling. The frequencies of the P${\mathrm{H}}_3$ lines and the mid-frequencies of the N${\mathrm{H}}_3$ doublets have been shown to agree very accurately with formulas of the type ${{\nu }_J}^{J-1\mathrm{}}=BJ-D{J}^3$. Determinations of the moments of inertia $A$ of N${\mathrm{H}}_3$ and P${\mathrm{H}}_3$ have been made and in the case of N${\mathrm{H}}_3$ the measurement of the doublet separation has led to a slight revision of the values for the moment of inertia $C$ and for the molecular dimensions obtained by Dennison and Uhlenbeck.